1. Field of the Invention
The present invention relates to an aluminum-alloy rolled sheet adapted for superplastic forming. More particularly, the present invention relates to an aluminum-alloy rolled sheet which can be preformed and then superplastically formed. In addition, the present invention relates to a method for producing such a preformable aluminum-alloy rolled sheet.
2. Description of Related Arts
In recent years, a variety of superplastic materials, which exhibit an exceedingly high elongation without incurring local distortion or necking when stretched at an appropriate strain rate at an elevated temperature, have been developed. Specifically, various studies of aluminum-alloy materials have been directed at those exhibiting superplastic properties at a temperature of 350.degree. C. or more, in terms of elongation of 150% or more.
Aluminum alloy can be readily formed into complicated shapes when conventional superplastic materials are used, such as Al-78% Zn alloy, Al-33% Cu alloy, Al-6% Cu-0.4% Zr alloy (Supral), Al-2.5.about.6.0% Mg-0.05.about.0.6% Zr alloy, Al-Zn-Mg-Cu alloys (AA 7474, AA 7075 etc) and the like.
The above mentioned, Al-2.5.about.6.0% Mg-0.05.about.0.6% Zr alloy belongs to a JIS 5000 series alloy, i.e., an Al-Mg based alloy, and is a static superplastic material.
The present assignee filed Japanese Patent Application No. 5-47431, in which it is disclosed that not only the above mentioned aluminum alloy but also other Al-Mg alloys can exhibit a static superplastic property provided that: the alloy composition is properly selected; and, the production process of the alloy is properly controlled in such a manner that the grain-diameter of recrystallized grains are very fine when the alloy is subjected to superplastic forming.
In Japanese Unexamined Patent Publication No. 3-89,893, corresponding to U.S. Patent application Ser. No. 07/711,308 filed by the present assignee, there is disclosed a superplastic forming aluminum alloy, which essentially consists of from 2.0 to 8.0% of Mg, from 0.3 to 1.5% of Mn, from 0.0001 to 0.01% of Be, an optional element selected from Cr, V, and Zr, an optional grain refining agent of Ti or Ti and B, less than 0.2% of Fe and less than 0.1% of Si as impurities, and the balance of Al, wherein the intermetallic compounds have a size of up to 20 .mu.m, and the hydrogen content is up to 0.35 cc/100 grams.
The superplastic materials, whose formability is excellent at elevated temperatures, can be used in various applications. As to the superplastic aluminum-based materials, they can be applied for complicated shaping of various structural parts of automobiles, electric trains, and other vehicles. In the structural application, an importance should be attached not only to the superplastic formability but also the strength.
The conventional aluminum-based superplastic forming materials can attain complicated shaping utilizing superplastic forming but involve a drawback in insufficient strength. More specifically, when the conventional materials are subjected to complicated forming only by superplastic forming, they are highly stretched locally, thereby decreasing the sheet thickness in a certain locality and thus incurring a structurally low-strength portion.